It almost goes without saying that the capacity to withstand distortion without breaking was the meaning of and the reason for the use of the term “malleable.” But wrought iron is malleable as also is mild steel, and, in Europe fifty years ago (though in general not now) by the term “malleable iron” was meant and understood what we know as wrought iron. You will remember that Bessemer’s paper announcing his great process was entitled “The Manufacture of Malleable Iron and Steel without Fuel.” The first reference was to wrought iron. Bessemer did not succeed in making this by his process but his success in the manufacture of steel was immense. Therefore, while in ordinary conversation such definiteness is not necessary, perhaps, and not usual here, to be safe one should say “malleable cast iron,” and not simply “malleable iron,” for by the latter, many Europeans still understand wrought iron.

Like “Topsy” of “Uncle Tom’s Cabin” fame, the various members of the iron family “just growed.” Therefore a strictly logical classification and nomenclature is hardly to be expected.

It was mentioned in a former article that a process for making malleable the brittle white cast iron was discovered, or at least described, by Reaumur, a Frenchman, about 1722. It is likely that his discovery or acquaintance with it came about through his extended experiments with cementation steel.

The publicity which Reaumur voluntarily gave to his researches forms a notable exception to the customs of those days when it was the usual thing for manufacturers jealously to guard all trade secrets. These were handed down from father to son or to others of close interest in the business. So aside from Reaumur’s announcements concerning malleable iron, few details of its manufacture came to light during the eighteenth century. Even during the hundred years which have just passed there have been few lines in which greater secrecy has been maintained both in Europe and America. During the last thirty years, only, has real scientific work been done to make known the reactions which occur during annealing and the real causes of the malleability.

The father of malleable iron in this country was Seth Boyden, of Newark, New Jersey, a very ingenious man who well deserves the monument erected in his honor by the citizens of the city, which is pardonably proud of him.

Boyden apparently had no knowledge of the existence in Europe of the malleableizing process, but after noticing that a piece of formerly brittle cast iron had become rather malleable, apparently through the action of heat, he set about making experiments to produce a malleable material which could be produced more cheaply than wrought iron. By melting in a forge pieces of pig iron and then annealing in a small furnace in his kitchen fireplace the bars which he cast from the melt, he had worked out by 1826 a process that produced cast iron which was malleable. In 1831 he started a foundry and made a thousand or more different articles for which there was demand, and, from this beginning, an immense industry developed in this country.

We must not forget that the malleable cast iron as produced in this country is an entirely separate and distinct thing from the European malleable iron, as will be shown later. So our immense industry is our own and not a copied one.

It is only certain members of the cast iron family that can be made malleable by proper heat treatment. Alloys No. 14 and No. 15 represent one of these alloys before and after the annealing process. While No. 14 was given as a typical analysis for white cast iron for malleableizing it must be understood that compositions can vary considerably without detriment from that given.

There is one thing, however, which is absolutely necessary and that is that all or practically all of the carbon of the alloy must be in the combined form previous to the annealing process. This means that the alloy shall be white cast iron and have no free graphite, for any graphite flakes will remain through and after the annealing process and weaken the alloy just as it weakens gray cast iron.

For producing this white cast iron two processes are in general use—the “cupola” and the “air furnace.” The latter predominates.

Operation of the cupola for malleable iron requires great skill and very close attention to detail, for, to malleableize easily and with the best results, the composition of the alloy must be regulated within narrow limits, very much narrower than for gray cast iron. However, this is entirely possible and cupolas are operated continuously for malleable cast iron for ten or more hours with very slight fluctuation.

In general, operation is very similar to that described for cast iron except that the composition of the charge is necessarily different, much lower silicon being required, and more coke has to be used for the melting.

Most malleable iron castings are made in sand molds, and, as stated, the iron poured must be of such composition and temperature that the castings so made will be white of fracture. It is possible to get a quick indication of the condition of the iron for pouring by making test pieces, every one in the same way, which, after cooling and breaking, will show by fracture the approximate composition of the metal. According to these test pieces, called “sprues,” which, at times, may be cast as often as every five or ten minutes, the mixture is regulated to produce a uniform product.

To illustrate: The fracture of a round sprue, or test piece, always ⅞ inch in diameter, when poured in the sand, cooled there to low red heat, quenched in water and then broken, should be white with only a few flecks of dark constituent. A gray iron fracture indicates too high silicon content and such iron is usually termed “low” iron. Castings of medium or heavy section, which, therefore, cool slowly in the sand, if poured of too high silicon, i.e., “low” iron, might precipitate a little graphite during cooling, even though thinner-sectioned castings which cool so much more rapidly would come white from the same iron.

While iron giving nearly white sprues is necessary for particularly large castings, to make sure that the usual run of malleable castings will come white in the sand requires very slightly mottled test sprues.

Test blocks also, with one side cast against an iron “chill” are poured to determine the depth of chilling, and test bars of various shapes are regularly made, to test after annealing, for tensile strength, torsion and other physical properties.

“Air furnaces” are much like longer puddling furnaces. They vary in capacity from ten to forty-five tons while occasionally small ones of as little as three or five tons capacity are met with.

The usual fuel is soft coal. The long flame passes from the grate at one end over the bridge wall and is deflected by the roof down upon the bath beneath. A chimney at the outgoing end furnishes draft. The furnace bed is usually of brick upon which is fritted (slightly fused) a mixture of sand with a little lime. In order to facilitate charging of the materials to be melted the roof is usually removable in parts, called “bungs.” These have frame work of iron which hold in place the fire bricks that come in contact with the flame. During charging these bungs are lifted off one at a time, and the iron materials are dumped through the openings. Small doors in the sides just above the bath allow “rabbling” or mixing of the charge and skimming of the slag which forms, and one or more spouts lined with fire bricks and clay provide for tapping out the metal when it is ready to pour.

Unlike puddling furnace and open-hearth no burning out of the silicon, manganese and carbon is desired, though, of course, some occurs and has to be allowed for in calculating the mixture. The intention is simply to melt together with the least possible loss a mixture of such materials as will give the average final composition which long experience has shown to give the proper qualities to the finished product.

Charges usually are of certain percentages of pig iron with not too much phosphorus, sprues from previous melts, more or less good malleable iron scrap and small amounts of steel scrap. These are melted down as quickly as possible. Occasionally the slag which accumulates is skimmed off and, after rabbling, test plugs are poured from the fractures of which the composition of the iron is judged.

When the silicon content is deemed proper or has been adjusted through longer action of the flame if too high or addition of more silicon in the form of a high silicon alloy if too low, the iron is tapped, provided it is hot enough.

Malleable iron is largely used for very small castings. These require very hot and fluid metal. So, even if it is of proper composition, the metal must be held in the furnace until it is of a high enough temperature to pour properly. Through prolonged and strong heating the iron may easily become oxidized or “burnt” and much skill is necessary for proper operation of the furnace.

After tapping, the iron must be got into the molds with the least possible delay.

As has been mentioned with former processes the melting of iron in contact with coke or coal results in more or less contamination with sulphur. For this reason cupola malleable has considerably higher sulphur than has malleable cast iron made in the air furnace. Cupola and air furnace each has certain advantages and certain disadvantages. While strength and elongation are somewhat greater in air furnace than in cupola malleable, both anneal well and give materials which are satisfactory for the purposes for which they are intended.

Cupola metal has an advantage in that the temperature and the composition can be closely maintained the same throughout the heat, perhaps more so than with the air furnace. With the latter the metal at the top of the bath is hotter than that underneath, and, through action of the flame and air, silicon is somewhat lowered before all of the heat can be poured, especially with air furnaces of large size. The metal can easily be “burnt” unless extreme care is taken. In the cupola we can get very hot iron continuously so that it is unnecessary to prolong the heat with the danger of burning that occurs with the air furnace.

Air furnace iron anneals rather more readily than does the product of the cupola, and the strength and malleability are usually greater. The former requires a temperature of about 1350° F., while the latter must have 1500° F., a difference of about 150° F. Whether this results alone from the somewhat higher sulphur of cupola malleable is not definitely known, but it is probable that, also, the slightly higher total carbon gives the iron-carbon chemical compound a tendency to persist more strongly.

The open-hearth furnace is sometimes used for making malleable cast iron. It melts much more quickly than does the air furnace which requires from three and one-half to nine hours per heat, depending upon the size. The quality of the product which the open-hearth furnace produces is of the best, but on account of the continuous operation necessary, this type of furnace is not largely used. Malleable iron has also been made in the Bessemer converter, and, occasionally, in the crucible furnace, but in this country the practice is not at all common. In Germany a great deal of malleable iron is made in the crucible furnace.

When dumped from the molds the castings are extremely brittle. The sprues are knocked off and the castings go to the “tumbling” mills where they are tumbled, either with the sprues, with hard iron (white iron) shot or star-shaped pieces of iron which quickly clean the sand from them and give smooth, clean surfaces.

At the chipping and sorting benches any remaining pieces of gates and other excrescences are removed while the castings are being handpicked and sorted. White iron, because of its brittleness, breaks easily and small protruding parts can more readily and cheaply be removed before annealing than after the castings have been thereby toughened.

Having the cleaned castings of white iron of proper composition, malleability is given to them by the heat treatment known as “annealing.” Through the influence of a cherry-red heat continued over a sufficiently long period the iron and carbon, which in the white iron are chemically combined, gradually become divorced, and, after complete annealing, the casting will be found to consist of free iron in which are imbedded throughout very small particles of coke-like carbon. Castings that before this heat treatment were so brittle that they broke into many pieces under a blow and so hard that they might scratch glass, are now found to be capable of withstanding considerable distortion without fracture and so softened that a needle may scratch them.

Not only is a proper temperature necessary for best annealing, but, as stated, a sufficient time must be allowed for the separation of the carbon and iron. The separation requires many hours and the cooling from the annealing temperature must be slow in order that the carbon and iron may not again unite, as they certainly would do were the castings chilled in water or otherwise cooled too fast.

Most manufacturers produce tonnage enough that castings have to be annealed in quantity. As iron at red or higher heat wastes very rapidly on account of scaling (in fact would take fire in air if hot enough), and the quality of castings deteriorates somewhat if even a small amount of air is allowed to come in contact with them during the annealing process, they are generally protected by enclosing in iron containers on which tops are luted and cracks filled with stiff, fire-resisting mud which keeps out the air.

These iron drums are of suitable size and shape that they can be stacked one upon the other to a height of from four to six feet. The stacks or “sets” of “saggers” as they are called, are run into large brick-lined retort chambers which are heated either with coal from a grate, by powdered coal, oil, producer gas or other fuel. The larger the furnace and the greater the tonnage of iron which must be heated, the longer will be the time necessary for bringing the furnace and castings up to the cherry-red heat which is necessary for the annealing. Therefore, the larger furnaces require a somewhat longer time than those of smaller size, though the time required for the annealing of the castings themselves is no longer. In this country with ordinary-sized furnaces the usual time for the annealing operation is approximately one week. This includes the heating of the furnace and castings and the cooling to a black heat again.

Some manufacturers, however, anneal without pots but they aim to have the castings protected from the flame and air.

Handling devices have been designed which facilitate loading and unloading the furnaces. With these the many sets of pots or saggers which the furnace holds can be very quickly charged or removed.

After shaking the castings out of the cooled pots, the dark coating is removed from them by “tumbling” with iron shot, pieces of leather or other polishing material in tumbling mills after which they are ready for any machining which may be necessary.

Photomicrographs Nos. 109, 110, 113 and 35 which show the samples at 75 diameters magnification show the course of the annealing process. No. 109 was taken from an unannealed casting. No. 110 was of the same iron after approximately thirty hours in the furnace. No. 113 shows that after about forty-five hours nearly all of the iron-carbon chemical compound has been broken down into black patches of free carbon, surrounded by the white areas of pure iron. After about sixty hours all of the iron and carbon have been divorced and the annealing operation is complete, as is shown by photomicrograph No. 35.

While heat alone effects the divorcing of the carbon and iron, which is the essential part of the annealing process, in the greater number of cases aid is given by what may be termed chemical means. Reaumur, who about 1722 discovered the annealing process, used iron oxide for the purpose. The white iron was packed in iron ore or mill scale. At the high temperatures employed the oxygen of the ore in some way not yet definitely known, gradually removed the greater amount of the carbon from the casting. It has always been a scientific conundrum how a solid, iron oxide, surrounding another solid, a piece of white iron, could remove from the latter its carbon when neither of them melts nor mingles with the other. Whether some of the oxygen from the ore penetrates the iron and burns out its carbon or whether the carbon of the casting itself migrates is not yet definitely settled. Certain it is that the carbon is gradually removed from the casting, from the surface first and with increasing length of time from greater depths.

In European practice malleable iron castings are still malleableized in this way, i. e., by burning out the carbon. The castings are made as thin as possible and the annealing in “packing” (iron ore or mill scale) is continued for from one to two weeks. At the expiration of this time the castings have a white, steely fracture which is entirely unlike the fractures of malleable iron castings which are made in this country. Photomicrographs of such malleable iron show few or none of the black spots which No. 35 exhibits, and analyses of castings annealed in this way give very low results for carbon.

While in this country the Reaumur process of annealing is not followed, a “packing” of ore or scale is generally used. Some use an inert packing such as sand, and as first mentioned, some use no packing at all. Really, one of the main purposes of the “packing” as now used is the prevention of warping of the castings in the pots while annealing. The annealing temperature is not so high as in Europe nor is the annealing continued so long, but when packing is used for the shorter time only, some surface carbon is removed and the carbon throughout the center portions of the castings is precipitated in the coke-like form which is known as “temper carbon” to distinguish it from graphite which is shown in photomicrograph No. 35. To the eye, then, fractures of such castings show black centers and white rims. They are known as “black heart” castings and these form the bulk of the malleable cast iron made in this country.

We may say, then, that there are in general three varieties of malleable cast iron: the “all black” which is annealed without “packing,” the “black heart,” annealed in “packing” and the most common kind in this country, and in Europe, but very rarely here, the “whiteheart” from which practically all of the carbon has been burned during the “anneal.”

Comparison of photomicrographs No. 35 and No. 30 given on page 181, will show at once one of the reasons for the much greater malleability of malleable cast iron. While the total carbon present is very nearly the same in the two irons, the difference in physical form causes great difference in the malleability of the two. In the gray cast iron, No. 30, the carbon is crystalline and in the form of long brittle flakes which cut through and separate the grains of iron. Thus “planes of cleavage” are formed which make the alloy unable to resist severe shock and cause it to be anything but malleable. It is not so with annealed malleable cast iron. Here the carbon is in the form of small pellets which are imbedded among the grains of pure iron, the malleability of which is not seriously impaired largely because of the continuity of the “pure iron” structure. A second reason for the ability of malleable cast iron to withstand shock is that in the burning out of the carbon of the outer portions of the casting very small cavities are left. These allow the surface to become considerably deformed and battered under successive shocks without great strain on the casting itself.

Nothing has been said so far concerning one trait of all of the irons and indeed of most metals and alloys which are used for casting purposes. This is the tendency to “shrink” during the solidification and cooling of the metal of the casting. On account of the freezing of the outer portions of the casting before the metal of the inside, there must result certain hollow places or cavities after the inside metal has cooled unless some channel is kept open through which fluid metal can pass inside to keep cavities from forming. We will not here go into the matter of shrinkage with its great worry to the molder nor the ingenuity and strategy through which he produces castings without shrinkage cavities. One of the methods taken to overcome the trouble will be explained in the chapter on Cast Steel which is to follow.

There is, however, another type of shrinkage—that exhibited by the contraction of the entire piece of metal as it gradually cools after solidification. This presents a rather curious and interesting case.

It is well known among founders and pattern makers, that gray cast iron shrinks during cooling about ⅛ inch per foot, white iron ¼ inch per foot and cast steel ⁵⁄₁₆ inch per foot. That is, a bar cast exactly one foot long will be found when cold to be ⅛ inch short if of gray cast iron, ¼ inch short if of white cast iron and ⁵⁄₁₆ inch short if of cast steel. The patterns have to be made larger than the castings desired to allow for this shrinkage.

But, during annealing, white cast iron loses one-half of its ¼ inch per foot shrinkage and the resulting malleable cast iron is found to have a net shrinkage of but ⅛ inch per foot which is the same as that of gray iron.

It appears that the precipitation of the temper carbon expands the bar throughout to practically the same dimensions which it would have had if flake graphite had been allowed to precipitate through slow cooling, as is the case with gray cast iron.

This is cleverly taken advantage of by manufacturers of swivels of malleable iron, such as those shown. The inner portions are separately cast first and thoroughly cleaned after which they are imbedded in another mold. The outer portions are then cast around them, shrinking so tightly upon the inner portions that they cannot be turned at all. However, upon annealing they loosen enough that they can readily be turned yet remain tight enough that they cannot be separated.

Malleable iron from which the carbon has not been removed can be hardened and given a steely fracture by sudden cooling from a red heat even if it has previously been annealed. Decarbonized malleable iron, also, can readily be recarbonized by the cementation process. These characteristics are often taken advantage of for the manufacture of tools from malleable iron. Hammers, wood working chisels, gears, etc., are quite largely made. Where they are sold at a cheaper price than the better steel tools and without misrepresentation, there can be little objection, but sometimes they pass for steel.

Ofttimes malleable iron castings are made in what are known as “permanent molds” of iron. They are really “chilled castings.” Annealing of these is accomplished in the regular way. Such castings have very smooth and beautiful surfaces but as the iron molds have high first cost they can be used only for castings for which the sales warrant the expense.

While much less malleable than is wrought iron or mild steel, annealed malleable cast iron has considerable malleability. It will resist great shock and can be severely battered and bent without breaking. It has about 75 per cent or more of the tensile strength of mild steel and because of the cheapness of its castings the malleable iron industry has developed wonderfully. About a million tons of this product are produced here each year.

Naturally malleable iron castings are used where a material with properties intermediate between cast iron and steel will suffice. Such are castings for railroad cars, for reapers, binders, and other agricultural machines, pipe fittings, and the cheaper grades of tools.